High-Field Anodizing Apparatus

ABSTRACT

Disclosed herein is a high-field anodizing apparatus, in which nanostructures are formed on a surface of a metal by immersing a metal anode and a counter electrode into an electrolyte charged in an anodizing cell to oxidize the metal, comprising: a power supply unit for applying a predetermined pattern of voltage between the metal anode and counter electrode in the electrolyte; a temperature control unit for maintaining the temperature of the metal anode, counter electrode and electrolyte constant; and a reaction rate control unit for measuring the value of current generated by the voltage supplied by the power supply unit and controlling the concentration of the electrolyte using the current value to maintain the current constant. The high-field anodizing apparatus is advantageous in that it is possible to prevent the damage of nanostructures caused by the rapid melting of a metal or the dielectric breakdown of an oxide film attributable to high-field anodization and to control the growth rate of nanostructures, thus greatly improving the productivity of nanostructures.

REFERENCE TO RELATED APPLICATIONS

This is a continuation of pending International Patent Application PCT/KR2009/007268 filed on Dec. 7, 2009, which designates the United States and claims priority of Korean Patent Application No. 10-2009-0093786 filed on Oct. 1, 2009, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a high-field anodizing apparatus for forming nanostructures on the surface of metal, and, more particularly, to a high-field anodizing apparatus which can prevent the damage of nanostructures and control the growth rate of nanostructures by controlling the reaction temperature and reaction rate of anodization.

BACKGROUND OF THE INVENTION

An anodizing process, which is a metal-surface treatment technology, has been widely used to prevent the corrosion of metal by forming an oxide film on the surface of metal or to color the surface of metal. However, recently, an anodizing process has been actively used to directly form nanostructures, such as nanodots, nanowires, nanotubes, nanorods and the like, or to make frames for forming nanostructures.

Al, Ti, Zr, Hf, Ta, Nb, W, and the like are known as metals which can be formed into nanostructures by anodization. Among these metal anodized films, an aluminum anodized film can be easily formed, can be relatively safely treated by electrolytes, and its nanopores and thickness can be easily controlled. Therefore, an aluminum anodized film has been frequently used to conduct research into nanotechnologies.

When aluminum is electrochemically anodized in an electrolyte solution including sulfuric acid, oxalic acid or phosphoric acid, a thick anodic oxide film is formed on the surface thereof. This anodic oxide film includes a porous layer formed by growing regularly-arranged pores in a direction from the outer surface of aluminum toward the inside of aluminum and a barrier layer formed by continuously forming pores by the oxidation of aluminum and the flowing of an oxide film [J. E. Houser, et al., Nat Mater. 8, 415-420 (2009)] at the boundary of aluminum and aluminum oxide.

In the anodic oxide film including the porous layer and the barrier layer, it is known that the distance (D_(int)) between pores, the size of pores and the thickness of the barrier layer are generally irrelevant to the kind of electrolyte or temperature, and are predominantly determined by applied voltage.

In the anodization of aluminum, mild anodization having a film growth rate of several micrometers (μm) per hour at a relatively low voltage and hard anodization having a film growth rate of several tens of micrometers (μm) per hour at a relatively high voltage are known. In the high-field anodization of the present invention, pores are rapidly grown and arranged at high voltage compared to the conventional hard anodization in aluminum surface treatment. In relation to the formation of nanostructrues, the typical mild anodization and high-field anodization, in which self-ordering occurs, are shown in Table 1 below.

TABLE 1 The conditions of mild anodization and high-field anodization in which self-ordering occurs Mild anodization High-field anodization interpore interpore Class. voltage distance voltage distance Electrolyte sulfuric acid 19~25 V¹⁾  50~65 nm 40~80 V^(4), 5))  90~140 nm oxalic acid 40 V²⁾ 100~110 nm 110~150 V^(6), 7)) 220~300 nm phosphoric acid 160~195 V³⁾ 405~500 nm — Film growth rate 2~6 μm/h 30~70 μm/h Current density 2~5 mA/cm² (constant) 30~250 mA/cm² (decreased with the passage of time) ¹⁾H. Masuda, et al., J. Electrochem. Soc. 144, L127-L130 (1997). ²⁾H. Masuda, et al., Science 268, 1466-1468 (1995). ³⁾H. Masuda, et al.,. Jpn. J. Appl. Phys. 37, L1340-L1342 (1998). ⁴⁾S. Chu, et al., Adv. Mater. 17, 2115-2119 (2005). ⁵⁾K Schwirn, et al., ACS nano 2, 302-310 (2008). ⁶⁾W. Lee, et al., Nat. Mater. 5, 741-747 (2006). ⁷⁾W. Lee, et al., European patent application EP 1884578A1, filed Jul. 31, 2006.

The interpore distance (D_(int)), which is the most important factor in aluminum nanostructures, is known to be about 2.5 nm/V in mild anodization and about 2.0 nm/V in high-field anodization. In the growth rate of an oxide film related to the formation rate of nanostructures, in the case of mild anodization, current density is maintained low (several mA/cm²), so that temperature does not rapidly increase at the interface between a metal and an oxide film, with the result that the dielectric breakdown of the oxide film can be prevented by only a simple cooling means such as a dual water jacket. However, in the case of high-field anodization, initial current density is very high (several hundreds of mA/cm²), so that the temperature of electrodes rapidly increases, with the result that a large electrolyzer is required in order to cool the electrodes [S. Chu, et al., Adv. Mater. 17, 2115-2119 (2005)] or an additional cooling means provided with cooling fins must be used [W. Lee, et al., Nat. Mater. 5, 741-747 (2006)]. Further, in the case of high-field anodization, when a high voltage of about 700V is applied, in order to prevent the dielectric breakdown of the oxide film, it is known that an electrolyte having still lower concentration than that (0.1˜0.5 mol/L) of a commonly-used electrolyte must be used [C. A. Grims, et al., US Patent Application 20030047505A1, filed Sep. 13, 2002].

Generally, in order to improve the pore alignment of an aluminum anodized film, a two-step anodizing method [H. Masuda, et al., Science 268, 1466-1468 (1995)] can be used. In the mild anodization, since an oxide film slowly grows, one or more days is required to form an easily-treatable membrane by removing oxide film and then oxidizing it. However, in the high-filed anodization, since pores are aligned within several tens of minutes due to great initial current, it is possible to obtain a nanomembrane having excellent pore alignment.

In order to form an anodic oxide film into a membrane, residual aluminum and a barrier layer must be removed. In this case, in order to remove the residual aluminum and the barrier layer, an electrochemical method and a chemical method are largely used. First, in the electrochemical method, the barrier layer is removed and then the residual aluminum is selectively melted by electrochemical reduction using voltage reduction and current voltage, and an oxide film is separated from the aluminum and then the barrier layer is suitably melted by pulse separation. In the chemical method, the residual aluminum is selectively melted, and then the barrier layer is melted. Meanwhile, the pore size of a membrane can be increased by suitably using a process of chemically separating a barrier layer, and the pore size thereof can also be decreased by coating the walls of pores using a chemical or physical process.

As such, although nanostructures having nanopores, the distance therebetween and the size thereof being able to be easily adjusted and the shapes thereof being uniform, are increasingly put to practical use, they are mostly used in the fundamental research of an anodic oxide film at a laboratory level, and it is actually insufficient to develop a process and apparatus for high-field anodization for mass-producing nanostructures or rapidly producing nanostructures. In order to produce nanostructures having excellent pore alignment using high-field anodization, it is necessary to maintain the temperature of a reaction interface constant and to lower the initial concentration of an electrolyte. However, in this case, there is a problem in that sufficient growth rate cannot be obtained because reaction rate is extremely slow.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a high-field anodizing apparatus which can prevent the damage of nanostructures and control the growth rate of nanostructures by controlling the reaction temperature and reaction rate of anodization.

In order to accomplish the above object, the present invention provides a high-field anodizing apparatus, in which nanostructures are formed on a surface of a metal by immersing a metal anode and a counter electrode into an electrolyte to electrochemically oxidize the metal, comprising: a power supply unit for applying a predetermined pattern of voltage between the metal anode 13 and counter electrode in the electrolyte; a temperature control unit for maintaining the temperature of the metal anode, counter electrode and electrolyte constant; and a reaction rate control unit for measuring the value of current generated by the voltage supplied by the power supply unit and controlling the concentration of the electrolyte using the current value to maintain the current constant.

In the high-field anodizing apparatus, the metal anode may be made of any one of Al, Ti, Zr, Hf, Ta, Nb, W, and alloys thereof, and may be pretreated by heat treatment, electrolytic grinding or chemical grinding.

Further, the counter electrode may have a tubular shape, and the electrolyte may be cooled by allowing cooling water to flow into the tubular counter electrode.

Further, the power supply unit may apply any one of direct voltage, alternating voltage, pulse voltage, bias voltage and combinations thereof between the metal anode and the counter electrode, and may control the voltage depending on the distances among pores of nanostructures.

Further, the temperature control unit may be provided at the rear side of the metal anode, and may comprise a temperature sensor and a cooling unit, and, if necessary, may further comprise a heating unit. In addition, the temperature control unit may further comprise an electrolyte cooling unit for lowering the temperature of the electrolyte.

Further, the reaction rate control unit may comprise: a measuring unit for measuring the current generated between the metal anode and the counter electrode by the voltage supplied by the power supply unit; and a high-concentration electrolyte supply unit which is opened when the value of current measured by the measuring unit is lower than the predetermined value thereof, and which is closed when the value of current measured by the measuring unit is higher than the predetermined value thereof.

The high-field anodizing apparatus according to the present invention is advantageous in that it is possible to prevent the rapid melting of metal caused by high-field anodization or the damage of nanostructures caused by dielectric breakdown of an oxide film, and in that the productivity of nanostructures can be greatly improved by controlling the growth rate of nanostructures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a high-field anodizing apparatus according to the present invention;

FIG. 2 shows the voltage-current-temperature curves at the time of electrolytic grinding and the surface of an aluminum material due to the electrolytic grinding;

FIG. 3 shows the voltage-current-temperature curves at the time of primary high-field anodization and the shape of an oxide film formed by the primary high-field anodization;

FIG. 4 shows the voltage-current-temperature curves at the time of secondary high-field anodization and the shape of an oxide film formed by the secondary high-field anodization using the reaction rate control unit of the present invention;

FIG. 5 shows the shape of an oxide film formed by pulse separation; and

FIG. 6 shows the final shape of nanostructures with their boundary layer removed and with their pores expanded.

REFERENCE NUMERALS

-   -   10: anodizing cell     -   11: electrolytic bath     -   12: electrolyte     -   13: anode     -   14: cathode     -   15: cathode lead wire     -   16: metal support     -   17: stirring unit     -   18: O-ring     -   19: cooling bed     -   100: power supply unit     -   200: temperature control unit     -   210: temperature sensor     -   220: cooling unit     -   230: heating unit     -   300: reaction rate control unit     -   310: measuring unit     -   320: high-concentration electrolyte supply unit

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a high-field anodizing apparatus for producing metal oxide nanostructures having regularly-aligned nanopores using high-field anodization. The high-field anodizing apparatus includes: a power supply unit for applying a predetermined pattern of voltage between a metal anode and a counter electrode in an electrolyte; a temperature control unit for maintaining the temperature of the metal anode, counter electrode and electrolyte constant; and a reaction rate control unit for measuring the value of current generated by the voltage supplied by the power supply unit and controlling the concentration of the electrolyte using the current value to maintain the current constant.

This high-field anodizing apparatus is advantageous in that it is possible to prevent the damage of nanostructures caused by the rapid melting of a metal or the dielectric breakdown of an oxide film attributable to high-field anodization and to control the growth rate of nanostructures, thus greatly improving the productivity of nanostructures.

The metal anode, which is an anode material, is made of any one of Al, Ti, Zr, Hf, Ta, Nb, W, and alloys thereof, and, if necessary, may be pretreated by heat treatment, electrolytic grinding or chemical grinding in order to make its texture uniform and flatten its surface. The counter electrode, which is a cathode material, is made of a carbon-based material such as graphite, carbon nanotubes or carbon black, or a conductive material such as platinum or stainless steel.

The electrolyte is selectively used depending on the kind of the anode material. When aluminum is used as the anode material, an aqueous sulfuric acid solution, an aqueous phosphoric acid solution, an aqueous chromic acid solution or a mixture thereof may be used as the electrolyte. When the temperature of the electrolyte must be decreased below zero, the electrolyte may be mixed with ethyleneglycol or the like and then used. Further, when titanium (Ti) or zirconium (Zr) is used as the anode material, a nonaqueous organic solution containing fluorine ions may be used as the electrolyte.

The power supply unit applies direct voltage, alternating voltage, pulse voltage and bias voltage between the metal anode and the counter electrode to form an oxide film on the surface of the metal anode. In this case, the power supply unit must be able to apply voltage depending on the distances among pores of nanostructures. That is, the power supply unit must be able to apply a direct voltage of 250 V or less or a pulse voltage of 700 V or less, and must have a current density of 500 mA or more per unit area (cm²) of a metal.

The temperature control unit comes into contact with the rear side of the metal anode, and includes a temperature sensor and a cooling unit for preventing the temperature of the metal anode from being increased above reference temperature, and, if necessary, may further include a heating unit for maintaining the temperature constant. Further, if necessary, the temperature control unit may include an electrolyte cooling unit in order to lower the electrolyte temperature that can be increased by a cathode reaction. The electrolyte cooling unit may supply cooling water into the counter electrode.

The reaction rate control unit includes: an analog or digital measuring unit for measuring the current generated between the metal anode and the counter electrode by the voltage supplied by the power supply unit; and a high-concentration electrolyte supply unit which is opened when the value of current measured by the measuring unit is lower than the predetermined value thereof, and which is closed when the value of current measured by the measuring unit is higher than the predetermined value thereof. Due to the reaction rate control unit, the value of current is maintained at a predetermined level, thus preventing the rapid melting of a metal or the dielectric breakdown of an oxide film. For this purpose, it is preferred that a voltage be applied to an electrolyte having low concentration in the initial stage.

In order to form a nanomembrane having an interpore distance of 280 nm in an oxalic acid solution, a high-field anodizing apparatus according to an embodiment of the present is configured as follows.

FIG. 1 is a view showing a high-field anodizing apparatus using a vertical-type anodizing cell. As shown in FIG. 1, the vertical-type anodizing cell 10, which is generally used when a large amount of gas is generated at electrodes, is configured such that an anode 13 is disposed on a metal support 16 connected to the (+) terminal of a power supply unit 100, and a cathode 14 is connected to the (−) terminal of the power supply unit 100 through a cathode lead wire 15. Further, the vertical-type anodizing cell 10 is configured such that an O-ring 18 is provided between an electrolytic bath 11 and the anode 13 not to allow an electrolyte 12 to leak out. Furthermore, the vertical-type anodizing cell 10 is provided with a stirring unit 17 such as an impeller in order to stir the electrolyte 12.

Due to the voltage supplied by the power supply unit 100, an oxide film formation reaction occurs at the anode 13, and a reduction reaction (electrolysis of water, etc.) occurs at the cathode 14. In this case, owing to these reactions, the temperature of the interface between the electrolyte 12 and each of the electrodes can be increased. Particularly, when the temperature of the anode 13 is increased above the predetermined temperature, the alignment of pores deteriorates. Therefore, in the high-field anodization of aluminum, the temperature of the anode 13 must be maintained at 0°. For this purpose, a cooling bed 19 is provided under the metal support disposed beneath the anode 13. The cooling bed 19 receives low-temperature liquid (0° or lower) from a circulator as a cooling unit 200 of a temperature control unit 200, and cools the lower portion of the metal support 16, thus absorbing the heat of the anode 13 by thermal conduction. For this purpose, a copper plate having excellent thermal conductivity may be used as the cooling bed.

As in the early stage of high-field anodization, when heat is excessively emitted from the anode 13, for the purpose of more precise temperature control, if the temperature of the anode 13 is maintained at 0° by the combination of cooling and heating by providing a temperature sensor 210 and a heating unit 230 in the metal support 16 instead of by decreasing the temperature of the circulator, rapid cooling can be performed by stopping the operation of the heating unit 230 at the time of emitting excessive heat. Such a system is very useful in the temperature control when the area of nanostructures expands.

Further, in order to decrease the temperature of the electrolyte 12, cooling water may flow into the cathode 14 using a metal tube-type counter electrode instead of a platinum net-type counter electrode which is generally used as the counter electrode, or a high-concentration electrolyte supply unit 320 may be used. That is, cooling water is supplied into the tubular counter electrode by an electrolyte cooling unit of the temperature control unit 200

Therefore, the temperature control unit 200 serves to decrease the temperature of the electrolyte 12 both by cooling the metal support 16 and thus absorbing the heat emitted from the anode 13, and by supplying cooling water into the counter electrode using the electrolyte cooling unit.

Meanwhile, in order to obtain nanostructures having excellent pore alignment, a two-step anodizing process of removing the oxide film formed by initial oxidation and then directly applying a voltage thereto or an imprinting process of forming regular pattern on the surface of an oxide film must be used. However, when secondary anodization is conducted in the high-concentration electrolyte (for example, 0.3 M of oxalic acid) used in primary anodization, the nanostructures are damaged due to the rapid melting of metal or the dielectric breakdown of an oxide film. This problem can be solved by conducting the secondary anodization in an electrolyte diluted to 1/100. Even in this case, since initial current is low and gradually decreased, desired growth rate can be obtained.

In order to solve the above problem, the reaction rate control unit 300 controls current such that the current is maintained above the current value measured by a measuring unit 310, and the high-concentration supply unit 320 supplies a high-concentration electrolyte. That is, the high-concentration electrolyte supply unit 320 is opened when the current value measured by the measuring unit 310 is lower than the predetermined current value thereof, and is closed when the current value measured by the measuring unit 310 is higher than the predetermined current value thereof. Since the current value is maintained at a constant level by the reaction rate control unit 300, the rapid melting of a metal or the dielectric breakdown of an oxide film by high-field anodization can be prevented. For this purpose, it is preferred that a voltage be applied to a low-concentration electrolyte in the early stage of high-field anodization.

FIG. 2 shows a photograph (FIG. 2A) of the sample obtained by electrolytically grinding an aluminum disk having a purity of 99.999% in a mixed solution of perchloric acid and ethanol of a volume ratio of 1:4 for 5 minutes, and shows a graph (FIG. 2B) of the change of voltage, current and sample temperature in this case.

FIG. 3 shows a photograph (FIG. 3A) of the primarily anodized oxide film obtained by increasing a voltage from 0 V to 140 V with respect to a platinum cathode under the conditions of a sample temperature of 0° C. and an oxalic acid solution of 0.3 M and then maintaining the voltage for 30 minutes, shows a graph (FIG. 3B) of the change of voltage, current and sample temperature in this case, shows a SEM photograph (FIG. 3C) of the initial pores of the lower portion of the oxide film, shows a SEM photograph (FIG. 3D, wherein aluminum was selectively removed by a mixed solution of copper chloride and hydrochloric acid) of the boundary layer of the lower portion of the oxide film, and shows a photograph (FIG. 3E, wherein an alumina film was selectively removed by a mixed solution of chromic acid and phosphoric acid) of the patterned surface of the remaining aluminum and a SEM photograph (FIG. 3E) thereof. In FIG. 3B, when an oxide film begins to be rapidly formed at a voltage range of 80˜90 V to obtain a maximum of current value and then the voltage reaches a constant voltage of 140 V, current is rapidly decreased by the diffusion control mechanism of an electrolyte. In contrast, when current is slowly decreased at a voltage of 140 V, the alignment of pores occurs. It is known that this pore alignment is improved with the increase of primary anodization time.

FIG. 4 show a photograph (FIG. 4A) of a secondarily anodized oxide film formed by directly applying a voltage of 140V in an oxalic acid solution of 0.003 M under the condition of a sample temperature of 0° C. to the sample obtained by selectively removing an alumina film from the primarily anodized oxide film, wherein current density was set to 15 mA/cm² and a high-concentration electrolyte was supplied when the current density fell below 15 mA/cm². Further, FIG. 4 shows a graph (FIG. 4B) of the change of voltage, current and sample temperature in this case. In FIG. 4B, the initial current density is rapidly decreased from 60 mA/cm², so that the initial current density is decreased at the level of mild anodization when a high-concentration electrolyte is not supplied. As shown in FIG. 4B, since current increases at the time of supplying an electrolyte, reaction rate can be controlled, and growth rate can also be controlled constantly or variably depending on time.

FIG. 5 show a photograph (FIG. 5A) of an oxide film obtained by applying a voltage of 150 V to the secondarily anodized oxide film in a mixed solution of perchloric acid and ethanol of a volume ratio of 1:1 by pulse separation, shows an obliquely-taken SEM photograph (FIG. 5B) thereof, and shows an SEM photograph (FIG. 5C) of the entire section thereof. It can be presumed from FIG. 5 that pores are grown in accordance with the pattern formed by primary anodization, thus forming an oxide film having a thickness of about 30 μm per hour.

FIG. 6 shows a photograph (FIG. 6A) of a final membrane obtained by removing a boundary layer using a 5% phosphoric acid solution and enlarging pores, and shows a SEM photograph (FIG. 6B) thereof.

As described above, the high-field anodizing apparatus of the present invention can be used to produce nanotemplate formed without separating an oxide film from a matrix material in addition to the nanomembrane. Furthermore, the high-field anodizing apparatus of the present invention can also be used to produce nanopores, nanowires, nanotubes, and the like.

INDUSTRIAL APPLICABILITY

The present invention relates to a high-field anodizing apparatus for forming nanostructures on the surface of metal, and, more particularly, to a high-field anodizing apparatus which can prevent the damage of nanostructures and control the growth rate of nanostructures by controlling the reaction temperature and reaction rate of anodization. 

1. A high-field anodizing apparatus, in which nanostructures are formed on a surface of a metal by immersing a metal anode and a counter electrode into an electrolyte charged in an anodizing cell to oxidize the metal, comprising: a power supply unit for applying a predetermined pattern of voltage between the metal anode and counter electrode in the electrolyte; a temperature control unit for maintaining the temperature of the metal anode, counter electrode and electrolyte constant; and a reaction rate control unit for measuring the value of current generated by the voltage supplied by the power supply unit and controlling the concentration of the electrolyte using the current value to maintain the current constant.
 2. The high-field anodizing apparatus according to claim 1, wherein the metal anode is made of any one of Al, Ti, Zr, Hf, Ta, Nb, W, and alloys thereof, and is pretreated by heat treatment, electrolytic grinding or chemical grinding.
 3. The high-field anodizing apparatus according to claim 1, wherein the counter electrode has a tubular shape.
 4. The high-field anodizing apparatus according to claim 3, wherein the electrolyte is cooled by allowing cooling water to flow into the tubular counter electrode.
 5. The high-field anodizing apparatus according to claim 1, wherein the power supply unit applies any one of direct voltage, alternating voltage, pulse voltage, bias voltage and combinations thereof between the metal anode and the counter electrode, and controls the voltage depending on the distances among pores of nanostructures.
 6. The high-field anodizing apparatus according to claim 1, wherein the temperature control unit is provided at the rear side of the metal anode, and comprises a temperature sensor and a cooling unit, and, if necessary, further comprises a heating unit.
 7. The high-field anodizing apparatus according to claim 6, wherein the temperature control unit further comprises an electrolyte cooling unit for lowering the temperature of the electrolyte.
 8. The high-field anodizing apparatus according to claim 1, wherein the reaction rate control unit comprises: a measuring unit for measuring the current generated between the metal anode and the counter electrode by the voltage supplied by the power supply unit; and a high-concentration electrolyte supply unit which is opened when the value of current measured by the measuring unit is lower than the predetermined value thereof, and which is closed when the value of current measured by the measuring unit is higher than the predetermined value thereof. 